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Author Topic: Balun for G5RV  (Read 3372 times)
AF5CC
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Posts: 941




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« on: May 10, 2013, 12:37:11 PM »

I have a pseudo-G5RV antenna that I homebrewed myself and want to do a little work on it since it has been around since 1998.  It is approximately 110 feet long, with radio shack 300ohm twin lead as the initial feedline. After 20 feet or so of the twinlead it makes the transition to coax.  Right now I have a 4:1 voltage balun at this transition because back in 1999 when I first installed the balun I didn't know as much about antennas as I do today, and figured going from 300ohm twinlead to 75 ohm coax (RG-11) required a 4:1 balun.

Anyways, I want to prevent radiation on the shield of the coax, so I guess I need a current balun at this transition, and a 1:1 would be fine, correct?  I am not interested in getting the antenna to resonate on most bands, because I know that it probably won't.  I used a manual tuner with it anyways. I spoke to a salesman at one respected antenna company today and he said for the G5RV you don't want a balun there because you need the feedline to radiate or something like that.  He suggested an in-line choke shortly before the coax goes to the rig.  I would prefer one at the twin lead to coax junction and have no radiation on the feedline.

So, for those of you who know far more about antennas than I do, what type of balun should I get and where should I install it?

73 John AF5CC
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WB6BYU
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Posts: 13341




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« Reply #1 on: May 10, 2013, 12:53:30 PM »

Quote from: AF5CC

Anyways, I want to prevent radiation on the shield of the coax, so I guess I need a current balun at this transition, and a 1:1 would be fine, correct?



Correct.



Quote

I spoke to a salesman at one respected antenna company today and he said for the G5RV you don't want a balun there because you need the feedline to radiate or something like that... 



So now you know not to believe whatever he tells you. 

The feedline on a G5RV shouldn't radiate:  it is a balanced line feeding a balanced
antenna.

Originally Lou Varney, G5RV himself, recommended against using a balun at the junction
due to the poor performance and understanding of baluns back in the 1950's.  By the
1960's or so he had changed his mind.  Some of the poor performance of G5RV antennas
is often due to power dissipated in the dirt due to common mode current along a coax
laying on the ground.
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AF5CC
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Posts: 941




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« Reply #2 on: May 12, 2013, 07:51:40 PM »

Thanks for the reply and the good information. It seems that there are two main types of baluns for the transition between ladder line and the coax. One is the type with the ferrite beads placed over a run of coax.  The other has wires wound around a torrid core.  Which of these would probably work better for my setup and purpose of keeping RF off of the coax shield?

73 John AF5CC
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WB6BYU
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« Reply #3 on: May 12, 2013, 09:40:04 PM »

Both of them work the same way:  one uses multiple turns through a single core,
and the other uses a single turn through multiple cores.

Looking at G3TXQ's measurements on different types of choke baluns here:
http://www.karinya.net/g3txq/chokes/, the ones using multiple turns on a single
core tend to have higher impedances than the ones using multiple beads on a
straight length of coax.
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WX7G
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« Reply #4 on: May 13, 2013, 09:54:54 AM »

A current balun using two separate cores will provide a high common-mode impedance and reduce radiation from the coaxial feedline.
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WB6BYU
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Posts: 13341




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« Reply #5 on: May 13, 2013, 10:03:50 AM »

Quote from: WX7G

A current balun using two separate cores will provide a high common-mode impedance and reduce radiation from the coaxial feedline.



A two-core design is required for a good 4 : 1 current balun, but in this application
a 1 : 1 is preferred, and that can be built satisfactorily with a single core.

Of course, you can always use two cores in series, optimized for different frequency
ranges, to get a wider useful bandwidth.
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KD0LAV
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Posts: 83




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« Reply #6 on: May 13, 2013, 11:46:41 AM »

I too am installing a new True Talk G5RV. Is a balan needed if a separate tuner from the radio is used?  I am new and there are so many different opinions I am soooo confused.
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KB6HRT
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Posts: 113




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« Reply #7 on: May 15, 2013, 06:36:57 PM »

Most G5RV that are installed do not use a balum, if you think you need one add about 21' of coax after the feed line and get your self a large Pepsi bottle and rap the 21' of extra coax around the bottle, then see if you see any difference, you never did say what you are trying to achieve, RF in your shack , hi swr or what. G5RVs work best when there in the open and about 36' off the ground with the open feed line stub away from metal an the G5RV can see 360 deg unobstructed, not alway possable but it will still work when that is not the case...........have FUN........thats what this hobby is about!...........KB6HRT
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WB6BYU
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« Reply #8 on: May 15, 2013, 07:04:58 PM »

You can use a G5RV without a balun, but you may lose power due to RF
flowing on the outside of the coax rather than into the antenna.  This is
true regardless of what sort of tuner you use.

The balun should be a 1 : 1 type between the balanced matching section
and the coax.  This is true regardless of the feedline length.
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G3TXQ
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Posts: 1524




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« Reply #9 on: May 16, 2013, 03:03:15 AM »

If you use a G5RV without a balun at the coax/ladderline transition there is the potential for significant braid current to flow; that may result in RFI problems and/or power loss on Tx, and increased noise pick-up on Rx.

The balun needs to be a 1:1 and have high choking impedance over the full frequency range of the G5RV - 80m thru 15m. That rules out a coiled-coax balun, which is far too narrowband.

Steve G3TXQ
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W5DXP
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« Reply #10 on: May 16, 2013, 05:01:34 AM »

The balun needs to be a 1:1 and have high choking impedance over the full frequency range of the G5RV - 80m thru 15m.

His particular "pseudo-G5RV antenna", i.e. 110' dipole fed with 20' of twinlead, only works well on 80m, 30m, and 20m. On all other HF bands, the SWR on the coax ranges from 15:1 to 72:1.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
AF5CC
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Posts: 941




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« Reply #11 on: May 19, 2013, 03:06:36 PM »

His particular "pseudo-G5RV antenna", i.e. 110' dipole fed with 20' of twinlead, only works well on 80m, 30m, and 20m. On all other HF bands, the SWR on the coax ranges from 15:1 to 72:1.

I am doing quite well with it on other bands, though.  Worked 8 band DXCC with it.  It is currently fed with RG-11 75ohm coax.  Would there be any advantage to using RG-8 52ohm coax instead?

John AF5CC
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AC2EU
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« Reply #12 on: May 19, 2013, 03:52:54 PM »

His particular "pseudo-G5RV antenna", i.e. 110' dipole fed with 20' of twinlead, only works well on 80m, 30m, and 20m. On all other HF bands, the SWR on the coax ranges from 15:1 to 72:1.

I am doing quite well with it on other bands, though.  Worked 8 band DXCC with it.  It is currently fed with RG-11 75ohm coax.  Would there be any advantage to using RG-8 52ohm coax instead?

John AF5CC


The impedance of a G5RV is all over the place,depending on the band, so a 1:1 current choke at the coax/ladder line interface is all that you can use. The tuner does the "heavy lifting " to make the match.
Since there is no "given" impedance, It probably won't make much difference between a 59 vs 75 ohm coax.
See http://www.karinya.net/g3txq/g5rv/

A resonant antenna is much better on receive and more efficient on transmit. One thing that I have discovered in my few years as a Ham, is that with the proper antenna setup, you can work the world with QRP.  As I click through my antennas to get to the resonant one for the band in use, stations that could not be heard at all suddenly are audible when I get to the resonant antenna.
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W5DXP
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« Reply #13 on: May 19, 2013, 07:22:00 PM »

It is currently fed with RG-11 75ohm coax.  Would there be any advantage to using RG-8 52ohm coax instead?

RG-8 would have no advantage over RG-11.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
VE7SHM
Member

Posts: 51




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« Reply #14 on: May 20, 2013, 07:34:45 AM »

Presently i am using a LDG 1.1 Balun it only rated for 200 watts but it did help lower my SWR very slightly ..I did buy and tried a easy tune 1.1 its rated for more power but it changed the SWR to I think a unacceptable level across 80, 40, and 20 meter.. I found a link on this forum to this site http://www.balundesigns.com/servlet/the-1-dsh-1-Isolation-fdsh-Choke-Baluns/Categories and ended up ordering this balun http://www.balundesigns.com/servlet/the-101/1-cln-1-Isolation-Balun-/Detail was recommend to me to work with the G5RV ....I am using a MFJ g5 at 38 feet i changed the down lead to 300 ohm twin line to help it match up better on 80 meters


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